USB Drive sometimes changes the Drive name

I have two USB Drives. I created partitions and formatted them to ext4. Running fdisk -l shows that I have /dev/sda1 and /dev/sdb1 Device Boot Start End Sectors Size Id Type /dev/sda1 /dev/sdb1 Then I mounted both on boot by running sudo nano /etc/fstab I use Samba to access them from the different computer and most of the time it is working fine, but sometime I can't access the files via samba and running fdisk -l shows the the device for my drives changed to for example /dev/sdc1. Sometimes it changes only per one drive and sometimes it changes both. I have no idea why this is happening. Any help is much appreciated.